SPF DNS Issues

Hi Guys
The server has been running great after the extra protection I put in place.
SPF is causing me some hassle, and I want to know how to properly respond to such issues.
Every now and again, we get domains that send emails from a seperate server to its parent domain.
Im sure that this is a configuration problem their end, but I don't know what to say 100% back to my clients as to why some emails are not getting through to them.